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Challenges in LED Research and Development: Current Droop

There's a problem that is often talked about with efficacy of LEDs, it's called droop. The higher the current density of the LED, the worse the performance. Droop refers to a reduction in efficiency of LEDs that occurs at very high currents, particularly in three nitride LEDs. There's been a lot of debate over the years exactly as to what the source of that is, but at this point, there's generally consensus that this is due to auger recombination. When we flow charge carriers into an LED, they have the option of either recombining and emitting light, which we call radiative, or recombining in a form that forms into heat, which would be non-radiative, and auger is one of the latter mechanisms.

So there's really two reasons that we want to eliminate droop, and part of this is application driven. So there's this desire to go to higher and higher flux, from a given chip. So we want very small, bright sources for many applications. And when we try to do that now, we run into droop, the efficiency begins to drop off. The other side of this is cost. If we have these very bright sources, we can have fewer of them, so at a system level we can reduce the cost overall. The thing about improving droop is that the gains that we would achieve are increasing at higher and higher current. So what we end up with is a situation where, we can run a device that used to be running at say, one amp, at some efficiency, now we can move that to 1.5 amps or higher, at the same efficiency. So the actual light output gain is tremendous.